The present invention relates to a thermosensitive recording material that can produce colored images therein by the application of heat, and further to a thermosensitive recording material that can exhibit high whiteness degree and excellent preservation stability of recording.
In general, a thermosensitive recording material comprises a support such as a sheet of paper or synthetic paper or a plastic film and a thermosensitive coloring layer formed on the support. The thermosensitive coloring layer comprises as the main components a coloring material such as electron-donating leuco dyes and a color developer, that is, an organic acidic material such as phenol compounds. The coloring material and the color developer are caused to react upon the application of heat energy thereto to produce recorded images. Such thermosensitive recording materials, which are disclosed in, for example, Japanese Patent Publications 43-4160, 45-14039 and 48-27736, have been widely put to practical use.
The thermosensitive recording material is used in a wide variety of fields, for example, for use with the output of an electronic calculator, a facsimile machine, an automatic ticket vending machine, a printer for scientific measuring equipment, and a printer for CRT medical equipment. This is because the thermosensitive recording material has the advantages that the recording apparatus therefor is compact, inexpensive, and simple in maintenance. However, it is known that colored images decolorize with time in the above-mentioned conventional dye-containing thermosensitive recording material prepared by providing a thermosensitive coloring layer comprising a coloring dye material, a color developer, and a binder agent as the effective components on a support. The reason for this is that the coloring reaction is reversible. Such de-colorization is accelerated by the exposure to light, high humidity, and high temperature. Further, the de-colorization proceeds more quickly to such an extent that the colored images cannot be read when the colored images are allowed to stand in water for a long time, or brought into contact with oils such as salad oil and plasticizers.
In addition, the above-mentioned dye-containing thermo-sensitive recording material spontaneously causes the coloring reaction when stored under the circumstances of high temperature and humidity, which coloring reaction is called coloring of background, because the recording material has the properties that the coloring reaction takes place by the application of heat to the recording material. Such coloring reaction of the background will decrease the contrast, so that the recorded images become illegible. The background causes a noticeable coloring reaction to make the recorded images illegible, in particular, when thermosensitive recording type parking tickets and passes are allowed to stand in vehicles under the blazing sun in summer, or when thermosensitive recording type POS labels attached to the packages of cooked food and packaged lunch boxes sold in supermarkets or convenience stores are heated in the microwave.
There are disclosed a variety of methods for reducing the above-mentioned decolorizing phenomenon, using a dye-containing coloring system comprising as a main component a colorless or light-colored lactone ring compound (e.g. in Japanese Patent Unexamined Publication (JP Kokai) 60-78782, 56-146796, 62-164579, 62-169681, and 62-19485). These methods, however, have little effect of improvement. Or, some methods show improved results to some extent, but such results cannot exhibit readily or last for a long time. In most cases, the obtained performance is still unsatisfactory. In addition, some methods for improvement cannot avoid the side effects of a coloring reaction in the background under the circumstances of high temperature and humidity or a decrease in recording sensitivity. A practical proposal has been thus desired.
As a countermeasure to solve the conventional problems from an utterly different viewpoint from the above, Japanese Patent Unexamined Publication (JP-Kokai) Hei 5-147357 and Hei 5-32601 disclose a method of using as the color developer sulfonylurea compounds instead of conventional organic acidic compounds such as phenols. This proposal to employ the sulfonylurea compounds as the color developer is epoch-making in such a sense that not only novel coloring functional groups are found, but also a thermosensitive recording material can be obtained with remarkably high preservation stability of recorded images. Namely, the thermosensitive recording material can succeed in complete control of de-colorization of recorded images formed in the thermosensitive recording material even when coming into contact with oils and plasticizers, which has been considered to be impossible in the prior art, as well as under various environmental conditions, for example, under the circumstances of high temperature and humidity.
However, by the presence of the above-mentioned sulfonylurea color developer, which can drastically improve the preservation stability of recorded images formed in the thermosensitive recording material, there has been an increasing demand for readily eliminating another drawback of the thermosensitive recording material, that is, the phenomenon that the recorded images become illegible because of the coloring of background to impair the contrast when the recording material is stored under the circumstances of high temperature and humidity. Namely, the proposal to use the sulfonylurea compounds as the color developer disclosed in Japanese Patent Unexamined Publication (JP Kokai) Hei 5-147357 and Hei 5-32601 does not always succeed in preventing the background from coloring under harsher environmental conditions, as currently desired, although the proposal has an effect on the improvement in preservation stability of the recorded images.
The coloring of background at high temperatures is an inevitable consequence of the properties that the thermosensitive recording material causes a coloring reaction by heating. However, owing to the improvement of the color developer and sensitizer it has become possible to reduce the coloring of background to lower levels (Japanese Patent Unexamined Publication (JP Kokai) 63-153183 and Hei 6-1069). Nevertheless, the thermosensitive recording material has been required in recent years to have the performance that recorded images can show such a striking contrast with no coloring of background that they can easily be read even though the environmental conditions become severer, for example, even when the thermosensitive recording type parking tickets and passes are allowed to stand in vehicles under the blazing sun in summer, or when the thermosensitive recording type POS labels attached to the packages of cooked food and packaged lunch boxes sold in supermarkets or convenience stores are heated in the microwave. This marks a turning point of great importance for the thermosensitive recording material.
As a proposal to meet the above-mentioned requirements, there is a method of using compounds with comparatively high molecular weight as the color developer (Japanese Patent Unexamined Publication (JP Kokai) Hei 8-333329). This method, however, is not acceptable for practical use because of low recording sensitivity although the method has a certain effect on the preservation stability of recorded images and the prevention of the background from coloring under the circumstances of high temperature and humidity.
No thermosensitive recording material is proposed that can be provided with both the capability of stability preserving recorded images and the capability of preventing the background from coloring under severer environmental conditions, and also with the fundamental properties satisfactory for a practical use.
The use of compounds having a specific similarity to the compounds of the present invention in structure as the color developer is disclosed in a thermosensitive recording material capable of repeatedly recording images and erasing the same by controlling the heating temperature of the recording material and the cooling speed thereof, which recording material is called a rewritable medium of thermosensitive recording type or a reversible thermosensitive recording material (Japanese Patent Unexamined Publication (JP-Kokai) Hei 7-257033). However, the rewritable thermosensitive recording medium of Japanese Patent Unexamined Publication (JP Kokai) Hei 7-257033 aims at more readily erasing the previously recorded images by the second application of thermal energy, so that it is essential that the color developer comprise a compound having a long-chain aliphatic group. The object and the guideline for designing and selecting the compounds used as the color developer in the above-mentioned application are utterly different from those in the present invention in consideration of one of the major objects of the present invention being to keep images from disappearing after once recorded even by the exposure to high heat.